A book scanner (BS) type image reading device and an auto document feeder (ADF) type image reading device are known as an image reading device mounted on a digital multifunction device. In the BS type image reading device, an original document placed on a glass is read by a scanning carriage with reading means. In the ADF type image reading device, the carriage is fixed and an original document is fed to read it.
When an original document is read by the ADF type image reading device, there is a possibility that, on the way of feeding the original document, foreign matter such as dust, paper powder, or the like which adheres to the original document is conveyed to a contact glass for ADF and the foreign matter adheres to the contact glass for ADF. When an original document is read in this state, there is a possibility that the read data includes the image of the foreign matter, and, as a result, a streak in a sub-scanning direction appears in the image and the quality of the image lowers. In both types of image reading devices, when foreign matter adheres to a mirror on an optical path of reading, there is a possibility that a streak in the sub-scanning direction appears like the above-mentioned case. If a streak appears when a user operates an image reading device, the user can remove the foreign matter if the foreign matter that generates the streak adheres to the contact glass. However, in case that foreign matter adheres to a mirror on the optical path, it is very difficult for the user to remove the foreign matter. Therefore, the user has to consume a maintenance service to remove the foreign matter. Accordingly, an inspection is performed before shipment. If a streak is detected, a cleaning is usually performed to remove foreign matter.
As a method for detecting a streak, the following methods are known for the ADF type image reading device. Reading means read data of a background plate through the contact glass for ADF in a state in which no foreign matter exists, and store the read data in a RAM (Random Access Memory). The reading means read data of the background plate before reading an arbitrarily original document, and by comparing the read data with the data stored in the RAM, detect a factor, such as the foreign matter on a stand glass or a mirror or the like, which generates a streak (refer to for example, patent document 1). Additionally, a method, which includes reading image data of an arbitrary original document by the ADF type reading means, finding the data which is not background data and is keeping up in the sub-scanning direction from the read image data, and detecting the data as the streak, is known (refer to for example, patent document 2). Further, as another method, a method, which includes reading a chart for inspection on which two stripes colored in black and gray, each of which has a constant gray level, are printed, transferring the read data to a personal computer (PC), and detecting the streak that appears on the stripes by an inspection program, is known (refer to for example, patent document 3).